Many electronic components, e.g. connectors, have a series of protruding pins that are used to connect such components to a printed circuit board (PCB) by way of pin through hole (PTH) barrels installed in the PCB. PTH barrels are generally “spool” shaped and include an annular sleeve defining a central aperture which extends through the thickness of the PCB and an upper and lower annular ring attached to the sleeve that extend over a portion of the upper and lower surfaces of the PCB in the vicinity of the sleeve. The interface of the rings and the sleeves of a PTH barrel is often referred to as a barrel “knee”. The PTH barrels are typically made from copper.
To attach the component to the PCB, the pins are aligned with respective PTH barrels, the pins are fed through the central apertures defined by the respective barrels, and the pins are then soldered to the barrels as is well known in the art. In order to decrease the time in soldering the pins, a wave solder machine may be used to effect the primary attachment of the component to the PCB. Often, however, certain ones of the solder joints require reworking and undergo a rework process. Reworking the solder joints can be accomplished using a solder fountain.
A solder fountain includes a solder pot that contains a quantity of molten solder and a pump for pumping the molten solder up through a flow well which provides a wave of liquid solder that is used to rework the solder joints of the PTH barrels. Unused solder is re-circulated to the solder pot using a spill mechanism. The pump is typically adjustable such that the flow rate of the solder can be controlled. The temperature of the molten solder can also be controlled in order for the solder in the solder wave to attain a particular temperature.
As is well know in the art, solder is a fusible metal alloy, often made of tin and lead, which is melted to join metallic surfaces. The use of lead in general has become increasingly undesirable, leading to much interest in lead-free solders. Such lead-free solders contain tin, copper, silver, and sometimes bismuth, indium, zinc, and other metals in varying amounts. The lead-free replacements for conventional solder have higher melting points, and often this leads to the need to re-engineer some components and materials used in electronic assemblies such as PCBs. Lead-free solder joints may produce mechanically weaker joints depending on service and manufacture conditions, which may lead to a decrease in reliability using such solders.
Of particular concern in PTH rework applications is copper (Cu) dissolution. Cu dissolution often occurs when the Cu rings of the PTH barrels come into contact with the molten solder alloy during the rework process. Cu dissolution results in a deterioration of the barrel such that the ring may become detached from the sleeve or in some cases be completely dissolved. The occurrence of Cu dissolution is not only prevalent in lead-free solder alloys, but also eutectic leaded solder alloys, although generally to a lesser extent. The combined change in alloy composition and corresponding higher melting point of the lead-free solder requires that the molten solder maintain a higher operating temperature, thus often accelerating the rate and consequently the extent of Cu dissolution. The barrel knee is typically the most vulnerable to Cu dissolution. As a result, the acceptable contact time for lead-free solder is generally much lower than for leaded solder. For example, studies have shown that a copper barrel knee can only withstand approximately 30 seconds of contact time using SAC405 lead-free alloy, whereas a copper barrel knee exposed to leaded solders is known, in many situations, to be able to withstand approximately 60-90 seconds of contact time before complete dissolution of the knee occurs on the same test vehicle.
It is therefore an object of the following to obviate or mitigate the above-mentioned disadvantages.